Zealot was the surname of the apostle Simon (Mt 10:4; Mk 3:18; Lk 6:15; Acts 1:13), to distinguish him from Simon Peter.

In the first two verses the KJV uses the name "Simon the Canaanite," perhaps a
transliteration of the Heb. Aram. qana'na, "zealot."

Zealots were fanatical defenders of the theocracy; and who, while taking
vengeance on those who wronged it, were themselves guilty of great excesses.

According to Josephus

The Zealots or Canaaneans were also a political group. They were
revolutionaries, frequently revolting, their restlessness resulted in the loss of 200,000
lives from the beginning of the century up to the actual outbreak of war in 66
AD. Extreme nationalists of this type had existed since the time of the
Maccabees, and one of Herod's achievements was to keep them well in hand. But after his
death, and especially at the census ordered by Augustus, rebellion broke out; in Judea, it was brought under control by the persuasion of the priests, but in Galilee two men, Judas and Sadduk by name, organized resistance. Judas founded a
religious sect sworn to "invincible liberty and to God as their only leader and
Lord." Acts 5:37 shows that the rebellion failed. A subgroup, the Sicarii, or
assassins, appeared a few years after the middle of the century, and represented
the extremists of an extreme movement. They held out for three years after the
fall of Jerusalem in 70 AD.